Iris not moving on old Cooke lens

[Richard Man]

Hello, I just got a 1920s 4x5 Cooke lens (I love the old Cooke so couldn't resist). It looks gorgeous, but now I have a problem: when I first got the lens, the aperture turned as it should, but after a few turns, it's stuck in the closed position. I think the adjustment screw needs adjusting, but not sure exactly what to do. The aperture ring moves freely, but the adjustment screw is not in the right position or something to drag the iris with it.

 

[MARTIE]

Sorry, is the situation currently, the aperture ring turns freely but the leafs themselves are jammed stuck?

Is it possible to gain access, do the front and rear cells remove easily?

 

[shutterfinger]

Remove the set screw, it should look like a pin on the aperture ring end. Remove the aperture ring, it should slip off. Look into the slot that the aperture operates in, there should be a hole in the operating ring in the slot opening, if it is not visible remove the lens cells and attempt to open the aperture with a round object that fits into the opening. When the pin hole is visible in the slot position it mid way, slip the aperture ring on and align the set screw hole with the aperture operating slot and install the set screw running it in until it stops.

 

[Richard Man]

Thanks! Unfortunately the aperture doesn't come off - well, not by "simple" means anyway. I am going to send it to SK Grimes for front mounting option anyway so I will let them deal with it.

Thanks again

 

[shutterfinger]

Apertures are two rings with the blades between. One ring is fixed and the other movable. The blades have is pin on each end. The fixed ring has a hole for the blade pin to pivot in, the movable ring a slot for the blade pin to move in. Barrel lens have the movable ring against a machined shoulder in the inside of the barrel with the blades then the fixed ring installed behind it and likely pined into the barrel with the pins flush with the barrel and ring edge. The standard mounting is from the rear of the lens. The movable ring has a lip with a pin hole about 3/32 to 1/8 inch in diameter for the aperture operating ring set pin to fit into. The aperture operating ring on the external of the barrel is either slip on against a lip or pressed on with a light force. The barrel has a slot for the aperture ring pin to slide in. The movable ring's movement is limited by the aperture blades. The pin hole in the movable ring will go past the slot ends by the diameter of the aperture operating ring hole.
Removing the lens cells and the set screw from the operating ring then opening the blades to mid way with a smooth round object then looking through the set screw hole under a bright light one might be able to align the operating ring hole with the movable ring hole then reinstal the set screw without moving anything. The set screw in your picture is about half out. The head of the set screw will be flush with the edge of the operating ring to slightly below the edge when fully seated.

The operating ring has grease between it and the barrel from the factory than can dry out and cause the aperture to be stiff in movement. I recommend not attempting to open the aperture as 9, 11, and 13 blade apertures are a major PITA to reassemble. Flushing with solvent or alcohol, applying a pin point drop of oil on the operating ring through the barrel slot, and applying extra fine powdered graphite along the blade ends frees the aperture up.

 

[Steve906]

Originally these were designed to run completely dry, unfortunately instead of cleaning some service people often did oil or use horrible graphite products. Its not that hard to dismantle if your that way incline and patient. I've done it many times, Cooke are about the most durable irises around. No lube should be on the blades and only a tiny bit of the right stuff possibly on the operating ring.